1. Field of the Invention
This invention relates to integrated circuits and more particularly to anti-clipping mixer circuits.
2. Description of the Relevant Art
Mixer circuits are well known for combining a plurality of input signals such as audio signals and providing an output signal indicative of the combination of input signals. In a typical circuit configuration, the plurality of input signals are provided through separate resistive paths to the input of an operational amplifier. The operational amplifier is provided with a resistive feedback path having an adjustable resistance to adjust the overall gain of the mixer circuit.
When designing a mixer circuit with discreet components, the operational amplifier can be chosen such that the combined input signals do not exceed a maximum-rated voltage level of the operational amplifier. In addition, the variable feedback resistance can be adjusted to reduce the overall output voltage of the combined signals. By selecting the appropriate operational amplifier and by adjusting the feedback resistance properly, clipping of the output signal can be prevented while maintaining a relatively high signal-to-noise ratio.
Problems arise, however, if such a mixer circuit is fabricated on a digital integrated circuit. Within the digital integrated circuit environment, the maximum voltage level is typically limited to approximately .+-.5 volts. Therefore, the input signals provided to the input resistive elements must have relatively low levels to assure that the maximum voltage of the operational amplifier is not exceeded. Unfortunately, this results in a poor signal-to-noise ratio.